1. Field of the Invention
The present invention relates generally to a method and apparatus for transmitting and processing a video image signal and in particular, to a method and apparatus for transmitting and, processing compressed video image signals.
2. Description of Related Art
Systems for recording video signals for high definition television (HDTV) are known. High definition television video signals are made up of either an R/G/B signal, wherein each component has a bandwidth of 30 MHz, or an Y/Pb/Pr signal, wherein the luminance signal Y has a bandwidth of 30 MHz and the chrominance signals Pb and Pr have bandwidths of 15 MHz each. To digitize such signals without aliasing, a VTR sample frequency must be at least twice the signal bandwidth. Typically, digital VTRs for HDTV sample the luminance signal Y at 74.25 MHz and each of the chrominance signals Pb and Pr at 37.125 MHz. If these signals are quantized with an eight-bit resolution, the full data rate for a digitalized HDTV signal is 1.188 Gbps. This is a significant data rate.
There are commercial VTRs that can record at this rate, however, this equipment is expensive, consumes a significant amount of electrical power, and requires a large amount of magnetic tape to hold this data. Thus, recording uncompressed HDTV video images is impractical for compact video/camera combinations.
Data compression methods are known that can reduce the volume of data required to record a digital HDTV video signal. It is possible, therefore, to construct an HDTV-compatible VTR using compression methods at an acceptable cost and power consumption rate.
To effectively reduce the data set required to reconstruct an HDTV video image, data compression methods typically involve some loss of image data. Image quality loss is generally confined to features of the image that are not readily distinguishable by human vision. The image resulting after decompression is visually acceptable as an HDTV image.
A problem arises when compressed video images are edited. The compressed video signal must be decompressed or expanded before it can be edited. The edited signal must then be compressed again for storage. Each compression/decompression cycle results in additional loss of signal information. As a result, the video image is degraded.
Signal formats for compressed and uncompressed video signals are not generally compatible. For a compressed video signal to be transferred from one VTR to another it must first be decompressed and then converted to a standard format. For HDTV, the BTA S-002 (bit parallel digital interface standard) or the BTA S-004 (bit serial digital interface standard) standards are typically used to transfer uncompressed video signals. The uncompressed signal is then transferred to the second VTR where it is compressed and recorded onto a recording medium. Again, additional compression/decompression cycles reduce the quality of the image and limit the utility of the editing system.
The system shown in FIG. 11 illustrates the operation of a conventional video recording system. When a digital image signal according to a standard format, for example, the BTA S-004 standard, is applied to a terminal 1, this digital image signal is converted into a parallel signal by a serial-to-parallel conversion circuit 2 and then applied to an encoder 3 that compresses the image data. The compressed image data are recorded by a recording/playing apparatus, such as a digital VTR 5.
To play back the image, a decoder 6 expands the digital compressed image signal from the VTR 5 and a parallel-to-serial conversion circuit 7 converts the expanded signal to a serial digital image signal that satisfies the interface standard.
If a signal process operation such as a copy operation is repeated by this digital signal process system, the image quality is degraded because of the repeated encoding and decoding processes.
Directly copying compressed image signals from one VTR to another would add to the complexity of the system. The compressed image data do not comply with the interface standards used for uncompressed images so that the compressed and uncompressed digital image signals must be treated differently. Handling two kinds of data signals adds complexity to the construction of a digital editing system.